Electric furnace



April 21, 1925.

1,534,12 o. coRADa ELECTRIC FURNACE Filevdan. l0, 1923 2 Sheets-Sheet 2 j2me/? Patentedpr. 21, 1925.v

UNITED STA 'OSWLD CORADI, O'FVZURICH, SWITZERLAND.

ELECTRIC FURNACE.

I Application tiled January 10, 1923. Serial 110.611,80.

To all whom z'tmay concern :v y vlBe it known that I, OswALD Comuna citizen of the Republic of Switzerland, re-

' siding at Zurich, Switzerland, have invented tnrough their lower certain new and useful Improvements in Electric Furnaces, 0f which the following isy a specification, reference being' had therein to the accompanying drawing.

The known electric furnaces of the resistance type having a resistor of granular carbonaceous material, which, for instance, are described in Der elektrische Ofen im Dienste der keramischen Gewerbe und der Glasund Quartzglaserzeugung, by J. Bronn, Halle a. S.. 1910, Monographien ber angewandte Elecktrochemie, vol. 34, present the following disadvantages:

The current consumption in knownfur naces is comparatively high. as the bulky massof the resistor, which shows the same temperature in all points of Aits cross-sectional area, has a large outer surface in comparison with the inner surface surrounding the heating chamber. so that the radiation of heat towards the outside is 1.5 to 2 times as large as the useful heat transmission towards the heating chamber; further in vertical furnaces, in which the resistor exceeds a given height, the resistance is not( uniform owing to the influence of the weight of the upper ower partscof such a vertical furnaceare therefore heated in a higher degree than the upper parts and a uniform temperature throughout a comparatively large heating chamber can therefore not be attained.

Electric furnaces for heating. tempering and smelting purposes have already been proposed (vide U. S. A. patentspecication No. 1,335,199, French specificatipn No. 329,822/5,508 and German speciicatmns No. 153,100 and 201,202) in which the electrodes which the current is supplied areso arranged that the path of thev current is parallel `to the circumference of the crosssection of the heating chamber, for .instance a muffie,y however, these known furnaces present the disadvantage of being hotter 1n part than in their upper part; which is a great drawback in furnaces for heating steel or for bakingt porcelain, viceramics, etc. A thoroughly uniform temperature. is reached with the furnace according i to the' present invention in,which the cir- @5 cumferences ofthe cross-sectional areas of the chamber to be heated, or the mums re- .tional area of the muiile.

spectively, or the length of the path of the current are varied in accordance with the varying resistances present in the resistor, which surrounds the chamber.

Several embodiments of theV present invention areillustrated by way of example on the accompanying drawings, in which:

Fig. 1 is a vertical longitudinal section y along line I-I of Fig. 2 of a first constructional example,

Fig. 2 is a cross-section along line II-II of Fig. 1.

Fig. 3 is a horizontal section through a second constructional example,

f Fig. 4 shows a vertical section of a third constructional example andA Fig. 5 is a horizontal section along line V-V of Fig. 4..

Figs. 6 and 7 illustrate a fourth constructional example in sections along line VI-VI of Fig. 7 and VII-VII of Fig. 6 respectively,

Fig'. 8 is a vertical section along line VIII-VIII of Fig. 9 of a fifth constructional example, p

Fig. 9 is a horizontal section along line IX-IX of Fig. 8 and Fig. 10 is a vertical section along line X-X of Fig. 9.

Fig. 11 is a modification of a detail of Fig. 8. arts of the resistor on the lower part, the Y Referring, to` the furnace illustrated in Figs. 1 and 2, 1 denotes a base plate, on which the upright walls 3 are',J supported. The walls consist of the ordinary refractory material, for instance fire-clay, and enclose a hollow chamber, in which a muiile 2 is inserted. The annular space` between the muliie2 and the vertical walls 3 is lled with a more or less inely divided carbonaceous material 7, theresistor acting as resistance. Into the resistor there are inserted two electrodes 5 and 6 insulated from each other b means of a block 4 of refractory materia These electrodes extend over the whole height of the resistor and over the whole width of its cross-section, i. e. from the wall 2 of the muiile -to lthe vertical wall 3 of the furnace. The furnace is covered by an 'annular plate 8 provided with a central open ingthat corresponds to the free cross-secl closed by a plate 9 of refractory material. The electrodes 5 and 6 may consist of iron, in case high'temperatures have to be obtained they are preferably made of carbon.

When the current is cut' in the resistor ia This opening is p mums takes plate.

heatedfwhich heat is transmitted to the jacket of the mule, from where the further transmission of heat to the interior of the lit is a well lrnown fact that the electric current has the tendency to choose the shortest path, therefore the carbonaceous particles in contact with the inutile forming the shortest path will beV heated to the greatest extent. rlhe heat in the resistor mass 'i' decreases towards the outside in accordance with the increasing path of the current, so that the carbonaceous particles in contact with the inner side of the Wall 3 of the furnace remain much cooler than the particles adjacent to the mume 2. ln this way the heat losses by radiation towards the outside are correspondingly smaller and the heat can he raised to the limit which the material of the' niule will stand without the outer face of the furnace walls and the cover plates l and 8 being subjected to the same high degrees of heat, as 1s the case with known furnaces of the resistance type. rlhis fact ensures an important saving in energy and a considerable increase in the life of a furnace. With the furnace illustrated in Figs. l and 2 a uniform temperature within the maille 2 is ensured owing to the shape of the mulie. The inutile 2 has at its lower end a larger circumference than at its upper end. lf now this increase in the circumference corresponds to the decrease in the resistance in the Vertical direction the Inutile is uniformly heated in' every part of its height which is an indispensable requirement with furnaces for heatin case hardening and tempering or annea ing purposes. lli/lith furnaces of this type a uniform temperature within the chamber to be heated, i. e., within the mulle 2, is absolutely necessary., whereas with ordinary furnaces for smelting purposes differences in temperature between the upper and the lower parts of the resistor can be neglected.

\ Fig. 8 shows a furnace according to the invention provided with three electrodes and adapted to be worked with three phase current.

ln Figs. l and 5 af heating furnace is illustrated which is of a rectangular section in a horizontal plane and in which the heat insulating material surrounding the furnace is provided with a lateral opening l0 in order to permit the placing of the objects to be treate into the furnace.

A horizontal furnace is illustrated in Figs. 6 and 7 in which the cylindrical mue 2 is arranged eccentrically in the cylindrical resistor 7, in order to compensate for the difference in the resistance occurring through the lower part of the mass of the resistor being compressed by the weight of the parts of the resistor.

Figs. 8, 9 and l@ represent upper furnace of the channel type to various points of which current' is supplied by means ofv electrodes 5 and in which furnace in order to attain a uniform temperature in all the points of the chamber ll to be heated the cross-sectional area of and the lengths of the paths of the current in the various horizontal layers of the resistor are so dimensioned that all the horizontal layers have the same resistance in spite of the differing density of the material. Two adjacent electrodes of the furnace illustrated in Figs. 8-10 may also he united to form one single electrode l2 as is shown in lilig. ll.

` l claim:

l. lin an electric furnace of the resistance type, a resistance element consisting of a loosely piled up material, the cross-sectional areas and the lengths of the path of the current in the dier'ent horizontal layers of the material being so chosen that all these horizontal layers of material present approximately the same resistance so that a uniform temperature will he a tained in .a chamher to he heated.

2. ln an electric furnace of the resistance type, a resistance element consisting of a granulous carbonaceous material, the crosssectional areas and the lengths of the path of the current in the dierent horizontal layers of the material heilig so chosen, that all these horizontal layers of material present approximately the same resistance so that a uniform temperature will he attained in a charnher to he heated.

3. lin an electric furnace of the resistance type, a resistance element consisting' of a granulous carhonaceous material the crosssectional areas and the lengths of the path of the current in the dierent horizontal layers of the material heine; so chosen hy arranging for the lengths of the circumference of a chamber to he heated and of the resistance element in such a way that all these horizontal layers of material present approzriinately the same resistance so that a uniformI tempearture will he ohtained in a chamher to loe heated.

4l. ln an electric furnace of the resistance type, the comhination with a shell of refractory material. of a shell of heat transmitting material, and a resistance element consisting1 of a granulous carhonaceous material filled into the intermediate space 'between t.said two shells, the shape of said shells he- .so chosen that cross-sectional areas and length of path of the current result in the dierent horizontal layers of resistance element that all these horizontal layers of material present approximately the same resistance so that a uniform temperature will he attained in a chercher to he heated.

5. ln an electric resistance furnace having a furnace chamber; a resistor of loosely piled material the charnher,

llli

mames i and electrodes contacting with the resistor,

the shape of the resistor and cross-sectional dimensions being chosen to present current paths in all horizontal layers of substantially the same resistance to produce a uniform temperature throughout the furnace chamber.

6. In an elect-ric resistor furnace having a furnace chamber; a resistor of lloose material surrounding the chamber, and electrodes contacting with the resistor, the cross-section of the resistor varying uniformly to produce a uniform resistance in substantially all horizontal layers.

7,. In an electric resistance furnace having 'a furnace chamber, a resistor surrounding said chamber and decreasing in cross-section from the bottom to the top of the chamlmr. 8. In an electric resistance furnace having a furnace chamber, a resistor surrounding said chamber, electrodes contacting with said resistor at its maximum longitudinal section and the resistor uniformly decreasing in section from the bottom to the `top thereof.

in testimony whereof I aix my signature.

OSWALD CORADE. 

